In order to reduce the landing distance of an aircraft, and also to increase the safety factor when the aircraft is landing on a wet or icy runway, thrust reversers are used on the jet engines in order to produce a braking thrust for slowing the craft. Such thrust reversers function to reverse the direction of the jet thrust, from a normally rearward direction used for propelling the craft, to a forward direction for slowing or braking the craft.
Thrust reversers are typically formed of reverser doors which are hingedly mounted on a fixed structure called a jet pipe, and usually form the final nozzle of the gas turbine engine. The trailing edge of the reverser doors must be fully scarfed in order to enable their deployment by hinged movement to a position in which the two doors abut against each other, either at their trailing edges or at abutment fittings.
French patent 2,348,371 and U.S. Pat. No. 4,129,269 illustrate the non-planar exit area of the nozzle when the reverser doors are in the stowed position. This exit area is often referred to as a "fish mouth" exit opening because of the shape of the opening when viewed from the side. Such a "fish mouth" opening is technically considered as non-planar. Such a fish-mouth or non-planar opening is commonly used for thrust reversers formed by hinged doors.
However, experience has shown that this shape rather significantly degrades the engine performance in a forward thrust mode of operation, and this degradation is caused by two combined factors. Firstly, there is a rather significant loss of nozzle efficiency because of efflux spillage sideways through the cut away portions created by the scarfs. Secondly, there is a significant thickness of the trailing edge of the reverser doors, leading to an excessive base area, in turn leading to a non-negligible base drag.
In addition, the thrust reversers according to these patents significantly change the inner flow lines of the jet stream through the engine, since the throat of the nozzle is (and must be) located upstream of the trailing edge of the reverser nozzle. Also, from the throat to the trailing edge of the reverser, the inner skin of the door has a cylindrical or divergent extension. Irrespective of the fishmouth exit, or the cut away portions created by the scarfs (which significantly degrade the engine nozzle performance), the structure extending aft of the throat of the nozzle increases the nozzle discharge coefficient at all engine pressure ratios. This in turn modifies the engine operating characteristics. Therefor, with this type of thrust reverser nozzle, it is necessary to modify the basic aircraft performance manual.
In British patent GB 2,168,298 A there is an attempt to eliminate the fishmouth opening of the thrust reverser doors when the thrust reverser is in the stowed position. Although the system described in this patent is an improvement over French patent 2,348,371 and U.S. Pat. No. 4,129,269, during the forward thrust operation of the nozzle, the structure nonetheless has drawbacks, as it does not completely eliminate the scarfs. Also, the trailing edge of the reverser doors has a significant thickness, meaning that the base drag of the proposed design is probably high.
French patent 86.09838 describes a system which differs from the present invention in that the reverser doors rotate forward of a fixed annular fairing. To do so, the pivot point of the doors, as described in this patent, must be located far forward of the trailing edge of the door and at a significant distance from the engine centerline to enable pivoting of the reverser doors inside the exhaust nozzle. The arrangement proposed in this design also improves the forward thrust performance of the engine with respect to the systems disclosed in French patent 2,348,968 and U.S. Pat. Nos. 4,129,269 and 3,550,855. However, the compatibility of the reverser with the engine is degraded because when the reverser doors are in the deployed position, they are closer to the primary and secondary flow exhaust nozzle, and therefore the engine surge margin is reduced in reverse.
Another prior art thrust reverser system utilizes what is commonly referred to as a "four bar" thrust reverser control system, and was designed by the Rohr Company and installed, for example, on the Boeing 737 and Douglas MD80 aircraft. In this arrangement, the thrust reverser doors do not form the flow line of the jet of the engine. In theory, this arrangement represents an improvement, as the exhaust of the nozzle is planar, but a significant drawback arises in that the thrust reverser doors rotate behind a fixed annular fairing which forms the nozzle exhaust when the reverser doors are stowed. This arrangement leads to a rather complex mechanism for driving the thrust reverser doors, and leads to installation of very large fairings to house the control system. Not only do these large fairings extend far aft of the exhaust flare of the nozzle when the reverser doors are stowed, but also alter very significantly the outer flow lines of the nacelle fairing of the jet engine. For these reasons, this system leads to a weight increase and more importantly a degradation of the aerodynamic characteristics in the forward thrust mode of operation.
French patent 88.14695 proposes another technique for eliminating the fishmouth opening. This system uses two movable flaps to partially close the fishmouth exit shape of the thrust reverser nozzle. However this system has the major drawback of requiring a specific actuation system for rotating the flaps out of the way of the thrust reverser doors when these doors are caused to deploy. For engines installed on the side of the rear fuselage of aircraft, it would be virtually impossible to apply this technique because of the lack of space available between the engine pylon and the reverser. Therefor, it would be impossible to deploy the inboard flap, and consequently the thrust reverser could not deploy.
The primary object of the present invention is to provide a thrust reverser which overcomes the disadvantages of existing thrust reverser systems.
Another object of the present invention is to provide a thrust reverser which forms the flow line of the jet of the engine and completely eliminates the fishmouth cutout of the exhaust nozzle when the reverser doors are in the stowed position. This means that when the reverser doors are stowed, the exit or outlet of the jet exhaust nozzle is planar, unlike the thrust reverser nozzle described in French patent 2,348,371 and U.S. Pat. No. 4,129,269.
A further object of the invention is to provide a thrust reverser arranged such that when the thrust reverser doors are commanded to deploy, the reverser doors drive the exhaust nozzle with them so that the reverser doors always remain upstream or forward of the exhaust nozzle structure.
Still another object of the invention is to provide a thrust reverser which does not alter the jet flow lines (usually defined by the engine manufacturer), so that the jet flow lines of the reverser nozzle according to the present invention are similar to the flow lines of a non-reversing nozzle. In addition, the base area of the trailing edge of the reverser nozzle need not be increased to accommodate the installation of the thrust reverser.